Aluminum alloy products with high resistance to pitting corrosion

ABSTRACT

Aluminum alloy products are described which combine both good strength and resistance to pitting corrosion. They are extruded from an aluminum alloy of the AA1000, AA3000 or AA8000 series containing about 0.001 to 0.3% zinc and about 0.001 to 0.03% titanium. The alloy may also contain 0.001 to 0.5% manganese and about 0.03 to 0.4% silicon. These products are particularly useful in the production of extruded products, such as heat exchanger tubing.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/085,590, filed May 15, 1998.

BACKGROUND OF THE INVENTION

A difficulty with the use of aluminum alloy products in corrosiveenvironments, such as heat exchanger tubing, is pitting corrosion. Oncesmall pits start to form, corrosion actively concentrates in the regionof the pits, so that perforation and failure of the alloy occurs muchmore rapidly than it would if the corrosion were more general.

Pitting corrosion is accelerated when there is a strong tendency towardssurface passivation due to the growth of a corrosion resistant oxidefilm over the vast majority of the tube surface. However, such a film isnever 100% intact due to the presence of discontinuities in the oxide,which are in some instances due to nonmetallic inclusions orintermetallics in the metal. With such a situation the passive areas arecathodic to any corrosion which may begin at the discontinuities. Withsuch a large cathode/anode area ratio, the dissolution rate at theactive sites is very rapid and the tube will perforate by pitting in 2-6days in the SWAAT test.

Anthony et al., U.S. Pat. No. 3,878,871, issued Apr. 22, 1975, describesa corrosion resistant aluminum alloy composite material comprising analuminum alloy core containing from 0.1 to 0.8% manganese and from 0.05to 0.5% silicon, and a layer of cladding material which is an aluminumalloy containing 0.8 to 1.2% manganese and 0.1 to 0.4% zinc.

Sircar, WO 97/46726, published Dec. 11, 1997 describes a corrosionresistant AA 3000 series aluminum alloy containing controlled amounts ofcopper, zinc and titanium. It has a titanium content of 0.03 to 0.30%,but this level of titanium raises the pressures required for extrusion,which will ultimately lower productivity.

It is an object of the present invention to develop an aluminum alloywhich can combine both strength and resistance to pitting corrosion. Itis a further object of the invention to provide an aluminum alloy whichis resistant to pitting corrosion and which is particularly useful inthe production of extruded products, such as heat exchanger tubing.

It is yet another object of the invention to provide an aluminum alloyresistant to pitting corrosion which is useful in the production ofsheet or plate products.

SUMMARY OF THE INVENTION

This invention relates to aluminum alloy products having high resistanceto pitting corrosion which are based on AA 1000, AA 3000 and AA 8000series of aluminum alloys. In one embodiment, they comprise extrudedproducts in which the alloys contain about 0.001 to 0.3% zinc and about0.001 to 0.03% titanium. In another embodiment, they comprise extruded,sheet or plate products in which the alloy contains about 0.001 to 0.5%manganese, about 0.03 to 0.4% silicon, about 0.001 to 0.3% zinc andabout 0.001 to 0.03% titanium. The Zn content is preferably in the rangeof about 0.05 to 0.2%.

By having the titanium content below 0.03%, the alloy has improvedextrudability. These alloys may be extruded to form tubing havingexcellent resistance to pitting corrosion or they may be rolled to formsheet or plate products also having excellent resistance to pittingcorrosion.

The presence of the Zn in the above alloy is to prevent passivation ofthe tube or sheet surface and its associated cathodic polarization,thereby eliminating the large discrepancy in cathodic and anodic areaswhich also removes the large local corrosion currents and anyconsequential pitting. This addition of the Zn does not significantlyaffect the work hardening characteristics, nor the modification ofexisting or formation of new intermetallic compounds, and therefore theextrusion pressures and overall extrudability are unaffected.

The addition of Zn to the aluminum alloys in accordance with the presentinvention has been found to be especially useful when added to aluminumalloys of the AA 3000 series, in which Mn is the dominant alloyingelement, e.g. AA 3102 or Alcan 30015. These alloys are widely used inautomotive air conditioner heat exchanger tubing. The use of Zn in thealloy according to this invention is also beneficial in aluminum alloysof the AA 1000 series as well as aluminum alloys of the AA 8000 serieswhich are based on Fe and Cu.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the attached drawings in which:

FIG. 1 is a photograph of two samples of the invention in 20 day SWAATtests; and

FIG. 2 is a photograph of two comparative samples after a 20 day SWAATtest.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

For the purpose of illustrating the present invention, 7 inch diametertest billets were cast from different 3000 series aluminum alloys, withand without the addition of Zn. The alloys used are described in Table1.

The casts MGL and MGM are comparative casts with a higher Ti content inthe range shown in WO 97/46726.

The billets were homogenized for four hours at 580° C., machined to 4inches diameter to allow fitting into a laboratory press and extruded toform tubing having a diameter of 0.25 inch and a wall thickness of 0.016inch. The tubes were then SWAAT tested for 20 days in order to ratetheir resistance to pitting corrosion. The results obtained are shown inFIGS. 1 and 2, with FIG. 1 showing the results for casts MIG and MIH andFIG. 2 showing the results for comparative casts MGM and MGL. Theseclearly illustrate the very significant improvement in pitting corrosiondue to the addition of Zn to the alloy. Casts MIG and MGL with low Zncontents both exhibit large pits in the tube which fully penetrate thewall. The results also show that this improvement is achievedindependent of whether or not an addition of 0.16% Ti is present.

TABLE 1 Cast Number Cu Fe Mg Mn Ni Si Ti Zn Cr Pb Sn B Ga V MIG 0.00270.4625 0 0.1107 0.0016 0.083 0.0089 0.0022 0.0003 0.0012 0 0.0012 0.00890.0046 MIH 0.0027 0.4597 0 0.1101 0.0016 0.0792 0.0088 0.1658 0.00030.0012 0 0.0012 0.0091 0.0046 MGL 0.0026 0.0651 0 0.2475 0.0015 0.06250.1565 0.0025 0.0002 0.0031 0 0.0014 0.0113 0.0099 MGM 0.0024 0.0684 00.2481 0.0014 0.0629 0.1607 0.1612 0.0002 0.0032 0 0.0014 0.0015 0.0102

EXAMPLE 2

Following the same procedure as described in Example 1, a series ofsample tubes were prepared using five different aluminum alloys havingzinc contents ranging from 0.0017% to 0.2430%. The alloys used and theresults obtained are described in Table 2.

TABLE 2 Perforation Cast during 40 day Number Fe Cu Mn Si Zn SWAAT test?MKJ 0.409 0.0013 0.234 0.074 0.0017 Yes (2/12) MKK 0.406 0.0013 0.2340.073 0.0211 No MKL 0.419 0.0013 0.235 0.074 0.0769 No MKM 0.425 0.00130.233 0.076 0.1520 No MKN 0.428 0.0013 0.236 0.074 0.2430 No

What is claimed is:
 1. An aluminum alloy extruded product consistingessentially of an aluminum alloy of the AA 1000, AA 3000 or AA 8000series containing about 0.05 to 0.3% zinc and about 0.001 to 0.03%titanium.
 2. An aluminum alloy extruded product according to claim 1which contains about 0.001 to 0.5% manganese and about 0.03 to 0.4%silicon.
 3. An aluminum alloy extruded product according to claim 2wherein the zinc is present in an amount of about 0.05 to 0.2%.
 4. Analuminum alloy extruded product according to claim 1 comprising anextruded heat exchanger tube.
 5. An aluminum alloy extrusion, sheet orplate product consisting essentially of an aluminum alloy of the AA1000, AA 3000 or AA 8000 series containing about 0.001 to 0.5%manganese, about 0.03 to 0.4% silicon, about 0.05 to 0.3% zinc and about0.001 to 0.03% titanium.
 6. An aluminum alloy product according to claim5 wherein the zinc is present in an amount of about 0.05 to 0.2%.